Maybe some of the euro 4 Enviro 400's that Stagecoach have could be upgraded? There are 51 of them according to the fleetlist on here, they were new 2007 and 2008. Just thinking this as the routes they are used on spend a lot of time in the city centre and other areas under the control of Newcastle City Council. In comparison an Omnicity used on the 11 (or Toonlink) service only briefly passes through Newcastle. Would probably bring the most benefit if a Newcastle Centre low emissions zone is planned to upgrade the buses which pass through the most.